A. Field of the Invention
The present disclosure generally relates to ferroelectric materials having improved fatigue and dielectric breakdown characteristics for use in nonvolatile memory and energy storage applications. The materials include a polymer blend that allows for the improved characteristics.
B. Description of Related Art
Memory systems are used for storage of data, program code, and/or other information in many electronic products, such as personal computer systems, embedded processor-based systems, video image processing circuits, portable phones, and the like. Important characteristics for a memory cell in electronic device are low cost, nonvolatility, high density, writability, low power, and high speed. Conventional memory solutions include Read Only Memory (ROM), Programmable Read only Memory (PROM), Electrically Programmable Memory (EPROM), Electrically Erasable Programmable Read Only Memory (EEPROM), Dynamic Random Access Memory (DRAM) and Static Random Access Memory (SRAM).
ROM is relatively low cost but cannot be rewritten. PROM can be electrically programmed but with only a single write cycle. EPROM has read cycles that are fast relative to ROM and PROM read cycles, but has relatively long erase times and reliability only over a few iterative read/write cycles. EEPROM (or “Flash”) is inexpensive, and has low power consumption but has long write cycles (ms) and low relative speed in comparison to DRAM or SRAM. Flash also has a finite number of read/write cycles leading to low long-term reliability. ROM, PROM, EPROM and EEPROM are all non-volatile, meaning that if power to the memory is interrupted the memory will retain the information stored in the memory cells.
DRAM stores charge on transistor gates that act as capacitors but must be electrically refreshed every few milliseconds complicating system design by requiring separate circuitry to “refresh” the memory contents before the capacitors discharge. SRAM does not need to be refreshed and is fast relative to DRAM, but has lower density and is more expensive relative to DRAM. Both SRAM and DRAM are volatile, meaning that if power to the memory is interrupted the memory will lose the information stored in the memory cells.
Consequently, existing technologies are either non-volatile but are not randomly accessible and have low density, high cost, and limited ability to allow multiples writes with high reliability of the circuit's function, or they are volatile and complicate system design or have low density. Some technologies have attempted to address these shortcomings including ferromagnetic RAM (FRAM) which utilize a ferromagnetic region of a ferroelectric capacitor to generate a nonvolatile memory cell.
These capacitors are fabricated using two parallel conductive plates separated by a ferroelectric polymer layer. The ferroelectric polymer layer is essentially a thin layer of insulating film which contains a permanent electrical polarization that can be reversed repeatedly, by an opposing electric field. As a result, the ferroelectric capacitor has two possible non-volatile states, which they can retain without electrical power, corresponding to the two binary logic levels in a digital memory. Ferroelectric capacitors frequently use a polyvinylidene fluoride (PVDF-TrFE) copolymer as the ferroelectric material due to its large polarization value and electrical and material properties.
Ferroelectric capacitors also provide energy-storing functionality. When a voltage is applied across the plates, the electric field in the ferroelectric displaces electric charges, and thus stores energy. The amount of energy stored by a ferroelectric capacitor depends on the dielectric constant of the insulating material and the dimensions (total area and thickness) of the film, such that in order to maximize the total amount of energy that a capacitor can accumulate, the dielectric constant and breakdown voltage of the film are maximized, and the thickness of the film minimized.
While ferroelectric capacitors address many of the important characteristics for a memory cell and energy storage, they often have poor durability. For example, fatigue, due to repeated switching cycles or stress cycles, is a primary damage mechanism for these capacitors. In a ferroelectric capacitor, fatigue is the phenomenon whereby the remnant polarization value is decreased by repeated switching and deterioration of ferroelectric polarization properties. In particular, injection of charges from electrodes are often trapped at crystallite boundaries and defects, thereby inhibiting ferroelectric switching and leading to higher fatigue rates.